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CHEVRON EMC 9 58420213 03/29 '00:31 NO.689 04/06 <br /> •1 YJ uu9/006 Wendy Wyels <br /> August 13, 1997 <br /> 1 Page 3 <br /> administrative record. Please request that Geomatrjx verify that tate text of the document is <br /> 1 correct where it states on page 10 of Appendix A that 5000 trials were performed in the <br /> Stochastic analysis. The electronic file given to DTSC indicated that 500 trials were <br /> Performed. The verification process performed by DTSC utilized runs of 10,000 trials, <br /> 1 ever, and essentially reproduced the printed results. <br /> 3. Use of the Refined Screening Levels <br /> 1 The document contains a comparison of individual chemicals detected in soils to the refined <br /> screening levels, V/Wle presenting no problem with regard to human health impact,please <br /> 1 be aware that such comparisons are potentially misleading and can result in public t, econcerto <br /> where none is warranted. <br /> 1 For DTSC projects, HERD is currently not approving use of any soil screening level <br /> calculations without some assurances as to how those levels are to be applied. It has been <br /> our experience that incorrect application of these levels (e.g., as target 1 performancc standards or, cleanup goals) has resulted in inconsistent. and unn <br /> little if any benefit to public]tcalth. ecessary expense with <br /> 1 For residential settings, the levels should reflect exposures to humans over any 1000 f 2 <br /> Parcel which could be reasonably superimposed on a site. For comrnercial/jndustria] <br /> settings, they should reflect worker exposure over sections of a real or hypothetical <br /> 1 facility which could reasonably be accessed, including occasional intrusion into <br /> subsurface soils for normal activities (e.g.,trenching for underground utilities) expected <br /> over time. <br /> 1 For soils, these levels should also reflect the conditions under which risk is assessed. For <br /> exposure of residents or workers to soils, average concentrations are used in the exposure <br /> 1 estimates. For empirical data (e.g. analytical results from random sampling) to be used in <br /> this context, these averages arc expressed as the 95% upper ro dsamence limit to be% UCL) <br /> ) <br /> Of the estimated arithmetic mean calculated using those samples, or the <br /> 1 detected concenisatjon, whichever maximum <br /> is leas. We consider human health-based remedial <br /> levels to be attained if random sampling results confirm, that the 95% UCL of these <br /> samples arc at or below the calculated screening levels, and that isolated spots of <br /> 1 Contamination above these levels do not represent acute or subchronic human health <br /> hazards. <br /> 1 If removal is deemed necessary, target concentrations presented for attainment of the <br /> remedial levels, depending on the distribution of site contaminant concentrations, may be <br /> above remedial levels. Therefore, OSA generally recommends that ncent a removal <br /> ' scheme is presented , regardless of the target concentration that isjustified, random <br /> confirmation sampling in order to assure that the 95% UCL of remaini <br /> concentrafions are at or below the health-based screening level. ng contaminant <br /> i <br /> i <br /> i <br />